Use of line characterization to configure physical layered devices

ABSTRACT

A method of optimizing filter performance through monitoring channel characteristics is provided. A signal enters a channel and a receiver receives the signal. The receiver includes a FIR filter to remove near-end transmitted interference and recover a far-end desired signal. The filter has storage elements configured as a shift registers to move the signal, multipliers to multiply the signal by a filter coefficient, an intermittent summer to combine the multiplied results into a replica of an interfering signal, a final summer to remove the replica from the receiver signal to provide direct and indirect monitoring of the signal, where direct monitoring includes time or frequency monitoring, and indirect monitoring includes monitoring signal to noise ratio, error magnitude or bit error rate. The filter is optimized according to monitoring and includes reducing a dynamic range, reducing bits of precision, reducing linearity, the filter, and reallocating the filter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is cross-referenced to and claims the benefit from U.S.Provisional Patent Application 60/900,180 filed Feb. 7, 2007, which ishereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates generally to networking. More particularly, theinvention relates to a method of optimizing filter performance throughmonitoring channel characteristics in a network.

BACKGROUND

Recent telecommunications technology has improved communicationefficacy. Advanced methods of communications require creative solutionsto problems encountered in implementing such advancements. Finiteimpulse response (FIR) filters are commonly used in high-speed datacommunications electronics for cancellation of interfering signals, suchas echo, near-end crosstalk (NEXT) and far-end crosstalk (FEXT). Belowis described a system for canceling echo.

A FIR filter is typically implemented by using a series of delays,multipliers, and adders to create the filter's output. The process ofselecting the filter's length and coefficients is in the filter design,where the design effort should result in a frequency response whichmeets the desired specifications, including ripple and transitionbandwidth, and optimize the filter's length and coefficients. The longerthe filter (more taps), the more finely the response can be tuned,however the longer the filter, the more resources (power, circuitry,noise margin) are required to meet the performance requirements.

Currently, common data transceiver design includes an echo cancellerthat removes the undesired echo signal by creating a replica of the echosignal and subtracting it from the far-end generated signal. This methodmakes use of the fact that the echo signal is a linear function of anear-end transmitted signal, i.e. a given system with a given transmitsignal produces a predictable echo signal. Therefore, a mathematicalmodel of the echo signal based on the near-end data signal can beaccurately built inside the echo canceller. However, often times thefilter must include numerous taps to accommodate multiple operationalcircumstances, resulting in a lengthy filter that is over-designed,underutilized and burdensome to the system.

The near-end data signal is transmitted through an Adaptive LinearFilter, which produces an echo estimate. This echo estimate is thensubtracted from the incoming signal, which is the sum of the desiredfar-end signal and the interfering echo signal, leaving the desiredsignal and any un-canceled echo. This data is recovered from thisresulting signal. The difference between the resulting signal and therecovered data is used as the measurement of the error between thecurrent and desired result, and can be used to adapt the filter'scoefficients. This process is repeated until the error signal isminimized, and the echo estimate matches the echo as close as possible.

By examining the coefficient results of the Adaptive Linear Filter, itis possible to reconfigure the filter so that it is sufficient to cancelthe interfering signal, but not to require the filter to have excessivedynamic range, unneeded bits of precision, excessive linearity orunneeded filter taps.

Accordingly, there is a need to develop a method of examining thecoefficient results of the Adaptive Linear Filter to reconfigure thefilter so that it is sufficient to cancel the interfering signal, butnot to require the filter to have excessive dynamic range, unneeded bitsof precision, excessive linearity or unneeded filter taps.

SUMMARY OF THE INVENTION

The current invention is a method of optimizing filter performancethrough monitoring channel characteristics. In one embodiment of theinvention, the method includes providing a signal into a channel andproviding a receiver to receive the signal. The receiver includes afinite impulse response (FIR) filter disposed to remove a transmittedinterfering signal from the signal in the channel and to recover a farend desired signal. The FIR filter includes a plurality of storageelements configured as a shift register to move the signal, multipliersto multiply the signal by a filter coefficient and provide a multipliedresult, an intermittent summer to combine the multiplied results into areplica of an interfering signal, a final summer to remove the replicafrom the receiver signal, providing direct monitoring of the signal. Thedirect monitoring includes time monitoring or frequency monitoring andproviding indirect monitoring of the signal. The indirect monitoringincludes signal to noise ratio (SNR) monitoring, error magnitudemonitoring or bit error rate (BER) monitoring. The current embodimentfurther includes optimizing the filter according to the monitoring. Theoptimization is selected from a group consisting of reducing a dynamicrange in the filter, reducing bits of precision in the filter, reducinglinearity of the filter, disabling the filter, and reallocating filter.

In one aspect of the invention, the FIR filter is an echo-cancelingfilter.

In another aspect of the invention, the FIR filter is a near-endcrosstalk canceling filter.

In a further aspect of the invention, the FIR filter is a far-endcrosstalk canceling filter.

According to another aspect of the invention, the storage element is aRAM cell.

BRIEF DESCRIPTION OF THE FIGURES

The objectives and advantages of the present invention will beunderstood by reading the following detailed description in conjunctionwith the drawing, in which:

FIG. 1 shows a full-duplex communication system according to the presentinvention.

FIG. 2 shows a block diagram of an echo canceling FIR filter accordingto the present invention.

FIG. 3 shows a plot of the impulse response at the receiver due to apulse from a transmitter according to the present invention.

FIG. 4 shows a modified FIR filter with unnecessary taps removedaccording to the present invention.

FIG. 5 shows a FIR filter with a programmable delay replacing middlefilter taps according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Although the following detailed description contains many specifics forthe purposes of illustration, anyone of ordinary skill in the art willreadily appreciate that many variations and alterations to the followingexemplary details are within the scope of the invention. Accordingly,the following preferred embodiment of the invention is set forth withoutany loss of generality to, and without imposing limitations upon, theclaimed invention.

By monitoring the characteristics of the channel, filters may beoptimized to improve performance. The monitoring may be direct, in theform of time of frequency domain parameters, or indirect, by measuringthe signal-to-noise ratio (SNR) of bit error rate (BER). Optimization ofthe filter may include reducing their required dynamic range, bits ofprecision, linearity or disabling them altogether. Further, limitedfilter resources may be reallocated to where they would be most useful,such as delaying finite impulse response (FIR) filter taps.

Referring to the figures, FIG. 1 shows a diagram of the preferredembodiment having a full-duplex communication system 100. The system 100includes transmitter TX1 102 and transmitter TX2 104, which sends dataTX Data1 106 and TX Data2 108 onto the channel 110. The receiverconsists of echo canceling FIR filters (112/114), which removes echosignals (115/119) from the transmitted signals (116/118) and recoversthe far end data (120/122). Data lines (124/126) are disposed to providedata (106/108) to the filters (112/114).

FIG. 2 shows a block diagram of a prior art Echo Canceling FIR filter200. The filter 200 includes Flip-Flops 202 configured as a shiftregister 204 to move the interfering data 205, multipliers 208 tomultiply the data 205 by a filter coefficient (C_(i)), a first summer210 to combine the various multiply results 212 into a replica of theinterfering signal 214 and a final summer 216 to remove the replica 214from the received signal 206 and provide a desired output signal 218.For an echo or NEXT canceller, the interfering data 205 is thetransmitted data and for a FEXT canceller, the interfering data 205 isthe received data. The received signal 206 consists of the desiredsignal as well as the interfering signal 205 that results from thesystem response.

In many systems, the echo results from imperfections at the transmitter.FIG. 3 shows a plot 300 of the receiver impulse response 302 due to atransmitter pulse signal 304. Here, the transmitted pulse 306 createsnear-end reflected signals 308, which may be small for some time as thesignal propagates, then a far-end reflected signal 310 is created. Thisfar-end reflection 310 is a function of the length of the channel asdefined by a number of baud periods 312, thus the echo canceller needsto have FIR filter taps sufficient to filter up to the maximum length ofthe channel.

According to the preferred embodiment of the invention, as shown in FIG.4, details of the channel response are used to reconfigure the filter(112/114) so that it is sufficient to cancel the echo to the desiredlevel but not to include any more taps than necessary. By examining theecho response in FIG. 3, it is evident that the FIR filter taps betweenthe near-end and far-end reflection 400 are not needed, so the FIRFilter (112/114) can be modified to remove unnecessary taps 400.

Because the flip-flops 202 remain in an on state, data (124/126) isrequired to be passed, however, the multipliers 204 and coefficientgenerators (not shown) can be removed. Some advantages here are in powersavings and improved performance since each FIR tap will introduce noiseinto the replicated signal 214, which will eventually end up degradingthe desired signal 120/122.

Some methods that are useful in determining which portions of the filtermay be disabled include examining the time domain response of thesignal. The time domain response examination can be done by using theFIR taps in an adaptive loop, then examine the resulting coefficientvalues. The coefficient values below a particular threshold can indicatethe FIR taps that are not needed. Another method can include selectivelydisabling taps and measuring the resulting noise on the desired signal.The taps having an impact on the noise level should be left in an onstate, and the taps without a measurable impact can be left off.Accordingly, taps may be selected in groups or individually, dependingon design constraints.

FIG. 5 shows an alternate embodiment of the invention, where shown is ablock diagram of a FIR filter (112/114) built with a programmable delay500. Here, rather than disabling unused filter taps, if it is determinedwhen the channel coefficients are measured, that the channel is not asignificant source of reflections, the FIR filter (112/114) can be builtwithout the middle filter taps 400 but rather with a programmable delay500. Further, other forms of interference may be cancelled in thismanner. Some of those include inter-symbol interference (ISI), near-endcrosstalk (NEXT) and far-end crosstalk (FEXT).

In another embodiment of the invention, the filter does not need toreside in the receiver. A filter can exist in the transmitter (notshown), combining the replica signal with the transmit signal such thatthe interference created is cancelled when the signal gets to thefar-end.

The present invention has now been described in accordance with severalexemplary embodiments, which are intended to be illustrative in allaspects, rather than restrictive. Thus, the present invention is capableof many variations in detailed implementation, which may be derived fromthe description contained herein by a person of ordinary skill in theart. All such variations are considered to be within the scope andspirit of the present invention as defined by the following claims andtheir legal equivalents.

1. A method of modifying a finite impulse response filter, the methodcomprising: receiving a signal through a communications channel;filtering the signal using the finite impulse response filter to atleast partially remove an interfering signal; monitoring acharacteristic of the communications channel; and optimizing the finiteimpulse response filter based, at least in part, on the characteristicof the communications channel; wherein the finite impulse responsefilter comprises taps, wherein individual ones of the taps areassociated with respective coefficients; and wherein said optimizing thefinite impulse response filter includes: selectively disabling selectedones of the taps; measuring noise output from the finite impulseresponse filter; and disabling individual taps of the selected ones ofthe taps having no measurable impact on the noise output from the finiteimpulse response filter.
 2. The method of claim 1, wherein thecharacteristic comprises a time domain response to the signal.
 3. Themethod of claim 1, wherein said monitoring the characteristic comprisesmeasuring a signal-to-noise ratio of the received signal.
 4. The methodof claim 1, wherein said monitoring the characteristic comprisesmeasuring a bit error rate.
 5. The method of claim 1, wherein the finiteimpulse response filter is configured to at least partially remove anecho signal from the received signal.
 6. The method of claim 1, whereinthe finite impulse response filter is configured to at least partiallyremove crosstalk from the received signal.
 7. The method of claim 1,wherein said selectively disabling comprises disabling a group of thetaps.
 8. The method of claim 1, wherein said selectively disablingcomprises disabling the selected ones of the taps individually.
 9. Themethod of claim 1, wherein the interfering signal comprises inter-symbolinterference.
 10. The method of claim 1, wherein the interfering signalcomprises near-end crosstalk.